[Feature] Allow defining unit tests in a directory within test-paths

[dbeatty10]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Currently, unit tests can only be defined in a directory that is included in the model-paths (which is models by default).

But it would make intuitive sense if they could also be defined within the test-paths -- they are tests after all!

Describe alternatives you've considered

No response

Who will this benefit?

Cases like in #11083

Are you interested in contributing this feature?

No response

Anything else?

See related documentation issues:

• Link to docs for model-paths configuration docs.getdbt.com#6604
• dbt looks for unit tests in the directories in the model-paths config docs.getdbt.com#6605